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Is it true that :

$((2^n+n^2) \in \mathbf{P} \land n \geq 3)\Rightarrow n\equiv 0 \pmod 3 $

I have checked this statement for the following consecutive values of $n$ : $3,9,15,21,33,2007,2127,3759$

Note that $2^n+n^2$ is special case of the form $2^n+k\cdot n$. One can easily show using Dirichlet's theorem that for any odd $n$ sequence $a_k=2^n+k\cdot n$ contains infinitely many primes.

Any idea how to prove or disprove statement above ?

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1 Answer 1

up vote 9 down vote accepted

If $n =1 \mod 6$ then $2^n+n^2 = 0 \mod 3$.

If $n =2 \mod 6$ then $2^n+n^2 = 0 \mod 2$.

If $n =4 \mod 6$ then $2^n+n^2 = 0 \mod 2$.

If $n =5 \mod 6$ then $2^n+n^2 = 0 \mod 3$.

P.S. Looking mod 6 is natural, since $2^n \mod 3$ repeats after 2 steps. So to decide what is $2^n \mod 3$, we need to know if $n$ is odd or even. Thus we need to look $\mod 2$ and $\mod 3$ at the same time....

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,very good answer,thanks...I have found this .I wasn't aware that this sequence is well known... –  pedja Nov 6 '11 at 19:14

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